1. Field of the Invention
This invention relates to fiber optic transmission lines and, more particularly, to devices for attachment to optical fibers for monitoring a predetermined portion of the light beam transmitted therein in both directions.
2. Description of the Prior Art
The field of fiber optics has progressed in a relatively few years from laboratory curiosities and decorative pieces to present-day systems of high sophistication for optical communication and data transmission. Optical fibers or light tubes are specially fabricated filaments which exhibit the property of transmitting light longitudinally along a flexible axis. Various materials which can be used in fabrication of optical fibers and the particular properties thereof are described in Derick et al U.S. Pat. No. 3,508,589 and, in further detail, in British Pat. No. 1,037,498, cited therein. Low-loss fiber optic taps are important components for fiber optic data links and data buses. This is true because it is desirable to be able to tap a portion of a signal propagating through an optical fiber without breaking or terminating the fiber, since fiber terminations add unwanted optical losses to the system and unfavorably increase the need for highly precise fiber splicing and interconnecting arrangements. Since fiber optic transmission lines having a large number of a signal taps are inherently power-starved, it is important to minimize excess losses associated with these components. Furthermore, it is desirable to have taps which can be fabricated so that the tape ratio (the power output of the tap divided by the power into the fiber in a given direction) can be conveniently tailored to the unique requirements of a given system. Efficient fiber optic taps have been reported previously whereby two fibers are cleaved, or ground and polished, at specific angles and butt joined. For example, see Karr et al., "Lightwave Fiber Tap", Applied Optics, Vol. 17, page 2215 (July 15, 1978) and Kuwahara et al., "A Semi-Transparent Mirror-Type Directional Coupler for Optical Fiber Applications," IEEE Transactions on Microwave Theory and Technique, Vol. 23, page 179 (January 1975). In these examples, the tap ratio is variable, either by changing the cleavage angle or by using materials with different indices of refraction between the cleaved surfaces. However, devices fabricated by such methods are quite fragile and cannot be easily reproduced with sufficient accuracy.
It has been demonstrated that when an optical fiber is bent in the form of an arc, there is an increased tendency for light to escape from the bent region in a radiation pattern which is primarily in the plane of the bend and which is directed away from the center of curvature. See, for example, Gambling et al., "Radiation From Curved Single-Mode Fibers," Electronics Letters, Vol. 12, page 567 (Oct. 14, 1976); and Goell et al. U.S. Pat. No. 3,982,123. The tendency for light to escape from the bent region of the fibers is enhanced when a flat region is lapped and polished into the fiber surface perpendicular to the radius of the bend in the fiber.
Polczynski U.S. Pat. No. 4,089,584 discloses optical taps coupled to a single optical fiber in a straight configuration with one planar side formed along the fiber core. McMahon U.S. Pat. No. 4,021,097 discloses an optical coupling apparatus in which a bundle of side-by-side optical fibers formed in a ribbon are curved, lapped and joined to an adjacent slab of light propagating material having a related refractive index. This slab wave guide is extremely thin (comparable to the thickness of the optical fibers) and therefore is difficult to fabricate and very fragile.
A specific combination of a photodiode sealed to a single optical fiber for supplying electrical signal characteristics of incident optical energy is disclosed in d'Auria et al. U.S. Pat. No.4,103,154.
The use of prisms for coupling light with optical transmission systems is well known as exemplified by U.S. Pat. Nos. 3,610,727 and 3,905,676 of Ulrich; 3,614,198 of Martin et al. and 3,901,582, 3,918,794 and 3,933,410 of Milton. Also, see U.S. Pat. No. 3,838,908 of Channin; Cooper "Coupler for Optical Data" IBM Technical Disclosure Bulletin, Vol. 16, No. 5 (October 1973) and Hsu et al. "Single Mode Optical Fiber Pick-Off Coupler"Applied Optics Vol. 15, No. 10 (October 1976). Lastly, an optical fiber system utilizing photodiodes, and prisms to separate signals supplied to the photodiodes is shown in U.S. Pat. No. 3,777,149 of Marcatili.